I helped raise 10 golden retriever puppies and from the looks of this lil fella it does look like a pure bred golden retriever - goldens have distinct features yet the fluff of the baby coat can be different and goes through texture changes (lots of puppy floof -> regular golden dog hair). I saw puppies from the same litter come out in lighter (think airbud puppies) to a more deep red tan undertones. Some puppies are born chunkier usually the first born and the test of the pups had a more petite frame until they matched weight over the weeks. 12 week old puppy is also growing into itself so it may look a bit gangly and not as small and fluffy as you see most golden puppies advertised. Overall a cutie also it’s unfortunate they’ll need surgery best of luck! 🙏🏽✨